Left hand XT thumbie w/rapid rise rear mech. ?

I know this sounds like an asinine question but... anyone try this? I'm looking to maybe put together a 3 - 5 spd drivetrain for my Karate Monkey. I've got an old left XT thumbie (lost the rear shifter ) and a rapid rise XT derailleur. Thoughts on if it'll work? I also have a "normal" XTR rear derailluer as well. I know I won't have any indexing of any sort which is fine, just wondering if the shifter will prevent the der. from moving the chain around on it's own. Any input is appreciated, I figured this would be the place to find someone who's given this idea a whirl. Thx!